Factual errors: Xiao-Mei would never be able to be a surrogate without having a child of her own first. All reputable surrogacy agencies in the US require their surrogates to have had at least one full-term, live birth before becoming a surrogate.
Errors in geography: Although the creators of the show keep the location of Wisteria Lane a secret, several clues have been given that contradict each other. For example, in the first season the restaurant Saddle Ranch is featured once, suggesting that the mystery location is either in California or Arizona, where the restaurant has locations. However, two families, the Applewhites and the Mayfairs had moved from Chicago suggesting that Fairview is near there. Those two cities are very far from each other.
Errors made by characters (possibly deliberate errors by the filmmakers): Susan's last name is at first pronounced "Meyer" occasionally by all characters, before it eventually changes to "Mayer" for the second season onward.